Hydraulic machinery commonly includes one or more main flow control valves, which may include directional control valves. A main flow control valve is a fluid valve that is operated directly or indirectly by an external input command.
Each main flow control valve may include a main flow control spool that is operated in response to the input command to control fluid flow and pressure to one or more associated hydraulic fluid receiving devices of the machinery. The hydraulic fluid receiving devices may include one or more hydraulic storage devices such as tanks or accumulators, hydraulic linear or rotary actuators, other hydraulic valves or subsystems, and/or any other devices that receive hydraulic fluid.
In examples, the machinery may include a plurality of main flow control valves for supplying and/or operating different hydraulic fluid receiving devices in a hydraulic system of the machinery. A main flow control valve and its associated controls (for example, an associated pressure compensator valve) may be incorporated into a valve housing, and each such valve assembly is referred to as a worksection. Worksections of the same or different configuration may be combined, for example in a side by side arrangement. A worksection combined with other sections (for example, other worksections, an inlet section, and an outlet section) may be referred to as an assembly of valve sections).
A worksection operates by controlling the cross-sectional area of a main flow control valve variable area orifice. In examples, the main control valve variable orifice is located in a fluid flow path extending between an inlet passage and an outlet passage, or workport. The inlet passage may be connected directly or indirectly to a source of fluid flow and pressure, and the outlet passage may be connected directly or indirectly to one or more of the fluid receiving devices. The flow through a given main valve orifice area is dependent upon the pressure drop across the orifice.
A pressure compensated worksection is a worksection that includes a pressure compensator valve arranged to maintain a substantially predetermined pressure drop across the main control valve variable orifice under normal operating flow conditions independently of the inlet or outlet pressure. By maintaining this substantially constant pressure drop across the orifice, a constant and repeatable flow rate through the orifice is achieved for any orifice area that is selected by the input command. The pressure drop may be controlled in part by a pressure compensator spool and by the force of a biasing device, such as a spring, acting directly or indirectly against the spool. Pressure compensated worksections such as those described above may, for example, be a pre-compensated working section including a pressure compensator valve located prior to (or upstream of) the main valve variable orifice.
Worksections may also include load-sense passages. The load-sense passages may be operably connected to provide (i.e., transmit) a pressure feedback signal from an outlet passage, which indicates the fluid pressure required by the fluid flow receiving device controlled by the valve. The load-sense passage may be operably connected to a load sensing variable displacement hydraulic pump or other load sensing source of pressure and flow (i.e. load sensing bypass compensator or a load sensing priority flow divider with controlled flow to the pressure compensated valve) to provide a feedback signal to the source. Further, an outlet passage's pressure feedback signal may be connected to the compensator spool of the associated working section. The compensator valve thus maintains the predetermined pressure drop by sensing the downstream (or outlet passage) and upstream pressures across the variable orifice which act on the pressure compensator spool biasing force end and opposite spool end, respectively.
Operation of a hydraulic system of mobile working devices of this type may involve lowering of a large negative or pulling load in a controlled manner. In examples, pilot-operated counterbalance or over-center valves could be used on the return side of the hydraulic fluid receiving devices for lowering a large negative load in a controlled manner. The counterbalance valves generate a preload or back-pressure in the return line that acts against the main drive pressure so as to maintain a positive load, which therefore remains controllable. In examples, pressure from the pump may be lost while operating a hydraulic cylinder even though flow continues. If a speed of a piston of the cylinder increases, pressure on one side of the cylinder (e.g., rod side) may drop and the counterbalance valve may then act to restrict the flow to controllably lower the load.
When the directional control valve is operating in a load-lowering mode, the pilot-operated counterbalance valve is opened by a pressurized pilot line. To protect both directions of motion of a fluid receiving device against a negative load, a counterbalance valve may be assigned to each of the ports of the fluid receiving device. Each counterbalance valve assigned to a particular port may then be controlled open via cross-over by the pressure present at the other port. In other words, a respective pressurized pilot line that, when pressurized, opens a counterbalance valve is connected to a supply line connected to the other port.
However, in examples, complex production engineering measures are required to access or tap into the cross-over pressure lines, which are required to open the counterbalance valves. Such complexity increases manufacturing costs of the valve assembly that includes the counterbalance valves.
Further, in examples, directional control valves having counterbalance valves assigned to each work port may include a shuttle valve or similar device that compares workport pressures and sends the higher load-sense signal to the pump. For example, such operation of the shuttle valve may help in cases where an outlet pressure from a hydraulic cylinder is higher than an inlet pressure to the cylinder, causing a vacuum to develop in the inlet side of the cylinder. In this case, the shuttle valve (or similar device) may move to send the higher of the two workport pressures to the pump so that the inlet side of the cylinder fills faster and prevents a vacuum from developing. However, a spool of the shuttle valve shifting back and forth may cause the pressures to fluctuate until the pressures stabilize between the supply and the return sides of the valve, which causes variability and fluctuation in the system operation. Further, the shuttle valve may add complexity and cost to the valve assembly.
It may thus be desirable to simplify construction of the valve assembly by having a different source for a pilot pressure, rather than a cross-over configuration, to open a counterbalance valve and reduce manufacturing costs.